Layer modulated smectic-$C$ phase in liquid crystals with a terminal hydroxyl group

We investigated local layer structures of the three smectic-$C$ phases (Sm$C$, Sm$C^{\prime }$, and Sm$C^{″}$) in a liquid crystal with the terminal hydroxyl group using high resolution and microbeam x-ray diffraction. It is found that Sm$C$ is the conventional Sm$C_1$ phase and Sm$C^{″}$ is the bilayer Sm$C_2$ phase. The Sm$C^{\prime }$ phase forms an in-plane modulation structure, so that this phase is the smectic-$C$ antiphase. From the Fourier transform infrared spectroscopy, it is suggested that the intermolecular hydrogen bonding is important to induce the Sm$C^{\prime }$ and Sm$C^{″}$ phases.

Figure 1

Two-dimensional small angle x-ray diffraction pattern of three Sm$C$ phases: (a) Sm$C$ (monolayer Sm$C_1$) at 106 °C, (b) Sm$C^{\prime }$ (modulated Sm$C$) at 103 °C, and (c) Sm$C^{″}$ (bilayer Sm$C_2$) at 96 °C. Rubbing direction is parallel to $z$, and the sample rotates around the axis to obtain the Bragg condition. Bottom figures are schematic images of the diffraction pattern.

Figure 2

Temperature dependence of the monolayer ($d_{02}$, closed symbol) and bilayer spacing ($d_{01}$, open symbol) in the Sm$A$, Sm$C$, Sm$C^{\prime }$, and Sm$C^{″}$ phases.

Figure 3

Chemical structure of I-7 (a) and three structural models in the modulated Sm$C$ phase (b)–(d). Arrows indicate molecules in order to distinguish the head and tail of molecules. Its head shows the terminal hydroxy group side, and note that it does not mean the direction of the dipole moment. The red line is the layer modulation (distribution of head or tail part of the molecules). (b) Layer modulation occurs normal to the molecular tilt plane ($x$-$z$ plane). (c) Layer modulation occurs parallel to the molecular tilt plane ($y$-$z$ plane). (d) Layer modulation occurs both parallel and normal to the molecular tilt plane.

Figure 4

Two-dimensional small (center top side) and wide (left side) angle microbeam x-ray diffraction pattern in the Sm$C^{\prime }$ phase obtained from the 25-μm-thick planar cell without an applied electric field (a) and under the application of a square wave electric field (10 kHz, ±20 V) (b). Right top figures of (a) and (b) are microphotographs of irradiated position (indicated by arrows). The white scale bar in the photographs indicates 50 μm. Right bottom figures of (a) and (b) show the 3D structure of the planar cells and the relation between incident x ray, rotation axis ω, and molecular coordinates; $z$ is the layer normal and the y-z plane is a tilting plane. (c) Two-dimensional small x-ray diffraction pattern by rotating the sample cell from the geometry of (b) a few degrees of ω as $z^{\prime }$ normal to the incident x ray.

Figure 5

Temperature dependence of the in-plane layer modulated periodic length obtained from ${\mathit{q}}_{\mathbf{10}}$ in Fig. 1. In this cell the phase transition temperatures from Sm$C$ to Sm$C^{\prime }$ and from Sm$C^{\prime }$ to Sm$C^{″}$ are 105.5 and 96.9 °C, respectively.

Figure 6

Temperature dependence of the wave number of an absorption peak around 3400 cm${}^{-1}$ corresponding to O-H vibration influenced by intermolecular hydrogen bonds of I-6 (open circle), I-7 (closed circle), and I-8 (open square).